Method of manufacturing high strength structures having cellular cores



July 29, 1969 D. A. RUSSELL ETAL 3,458,608

METHOD OF MANUFACTURING HIGH STRENGTH STRUCTURES HAVING CELLULAR CORESFiled July 19, 1966 5 Sheets-Sheet 1 INVENTORS .Unwd /7. Russell[/Iesfafi E [ynuskl BY 5 Mu ATTORNEY o. A. RUSSELL E 3,453,608

MANUFACTURING HIGH STRENGTH RES HAVING CELLULAR CORES 5 Sheets-Sheet 2INVENTOR5 u 52// [ynask/ @012. Pa TTORNEY P Q. m9 0 O OQ WO 90 m o Unu wwov R 0 M 3 H d 3 DU -NWH METHOD OF STRUCTU i v wuqhh m wwvnhm NNQQK.wuenhw July 29, 1969.

Filed July 1 9, 1966 United States Patent 3,458,608 METHOD OFMANUFACTURING HIGH STRENGTH STRUCTURES HAVING CELLULAR CORES David A.Russell, Wilbraham, Mass., and Chester F.

Cynoski, Thompsonville, Conn., assignors to Monsanto Company, St. Louis,Mo., a corporation of Delaware Filed July 19, 1966, Ser. No. 566,371Int. Cl. B29h 7/20; B32b 7/02 US. Cl. 264-45 9 Claims ABSTRACT OF THEDISCLOSURE A resinous material is subjected to a foaming medium toeffect a partial absorption of the medium into the body of the resin.This is followed by heat treating the modified resin to develop anintegral foam surface thereon under heat and pressure, releasing thepressure to allow expansion of the composite structure and thenrecompressing the resin composite to form an integral resinous bodywherein voids are essentially eliminated and the cells are subdividedand oriented in elongated form normal to the solid portion of theresinous body.

This invention relates to a novel method for preparing cellular corestructures. The application of Edgar E. Hardy and David A. Russell, Ser.No. 272,540, filed Apr. 12, 1963, now Patent No. 3,262,625, describesand claims a plastic article of manufacture having an improvedcombination of insulating and structural characteristics composed of apartially foamed thermoplastic material. The process in that applicationbroadly involves (l) shaping a solid plastic form, (2) steeping theplastic form in a medium absorbable by the plastic for a predeterminedinterval of time, removing the plastic form from the medium and finallyheating the plastic to produce a foamed covering at the area of contactwith the absorbable medium which extends partially into the solidplastic form. The resultant plastic article of manufacture has asurprisingly improved combination of insulating and structuralcharacteristics wherein the insulating or foamed layer is integrallyattached to the solid substrate which gives structural strength to thecombination. Application Ser. No. 548.905, filed May 10, 1966 by thesame inventors, describes and claims a sandwich structure comprising alaminate formed by exposing a plastic surface to a medium absorbable bythe plastic for a predetermined interval of time, intimately contactingat least a portion of said surface with a second plastic surface andsubjecting the resulting assembly to elevated temperatures to partiallyfoam the first plastic element of the combination and at the same timeintegrally attach the same to the inner surface of the second plasticelement in the combination.

It has been observed in the structures of the inventions described inthe aforesaid applications that at minimum cycles, structures made bypartial steeping and foaming often give coarse cells and large voids inthe cellular composition of the foam. It has been discovered inaccordance with the invention that without increasing the total cycletime voids can be essentially eliminated and cell structure improved bya multiple expansion of the partially foamed plastic articles.Furthermore, it has been also found in accordance with the inventionthat greatly improved internal cellular formations may be produced witha very substantial reduction in the cycle time.

3,458,608 Patented July 29, 1969 A primary object of the invention isaccordingly to effect a very substantial improvement in physicalproperties of synthetic resin composite structures formed by partialfoaming procedures wherein a resinous material is subjected to a foamingmedium for a predetermined period of time to effect a partial absorptionof the medium into the body of the resin, followed by heat treatment ofthe modified resin to develop an integral foam surface thereon underheat and pressure, releasing the pressure to allow expansion of thecomposite structure and then recompressing the resin composite from anintegral resinous body wherein voids are essentially eliminated and thecells are sub-divided and oriented in elongated form normal to the solidportion of the resinous body.

It has been observed that in the multiple re-expansion procedure setforth above that coarse cells and large voids are essentially eliminatedand the cells become more elongated and oriented in a directionperpendicular or normal to the nnfoamed surface of the resin.Preferably, during the ordinary heat treatment described in theaforementioned applications (near the end of the normal cycle), thepressure is released to a predetermined set opening and the system isallowed to expand. For example, the expansion allowed between successivestages of pressure may be sufficient to allow the material to expandfrom about 10 to percent of the final thickness. This will dependlargely upon the type of thermoplastic material being handled in theprocess and the qualities desired in the final end product. Thisintermediate expansion period may be followed by further application ofpressure to recompress the partially foamed plastic article to anydesired extent ranging from about 10 to 100 percent of the initialthickness of the original foam thickness.

Referring to the drawings, it will be observed that FIGURE 1 illustratesin elevation the orientation of cell structure produced by the normalpartially steeping procedure while FIGURE 2 shows a plan view of howthese cells appear from above. FIGURE 3 is an elevation of the modifiedfoam area showing detailed cell orientation of a partially steepedthermoplastic resin prepared in accordance with the invention. FIGURE 4is a plan view from above similar to FIGURE ,2 of the cell structureproduced wherein the cells are virtually oriented in accordance with theprocedure of the invention.

FIGURE 5 shows the stepwise compression of a twoply, internallypartially foamed sandwich structure for purposes of illustration. It canbe understood, of course, that the principles of the invention arelikewise applicable to the formation of sandwich structures comprisingthree or more plies of partially foamed resinous materials in multiplecompression procedures. Stage 1 signifies the compression and partialfoaming of the sandwich structure containing the pneumatogen. Stage 2shows the withdrawal of the compression members to allow expansion ofthe partially foamed construction to any desired extent. Stage 3 showsthe recompression of the partially foamed and expanded structure whileStage 4 illustrates the further withdrawal of the compression members toallow expansion of the internal partially foamed structure, and, as canbe seen, the elongation and orientation of the cell structure in adirection essentially normal toward the opposite faces of the sandwichconstruction. This can be followed, of course, with succeeding expansionand recompression stages to any extent desired to produce specialeffects in the finished article. FIGURE 6 shows the recompressionprocedure of my invention applied as a continuous process, wherein thepneumatogenic agent is applied, in this instance, to the internal facesof the thermoplastic so that the said reagent is absorbed into the interior of the opposing faces of the sandwich structure. It is shown inthe continuous process illustrated by FIG- URE 6 the heating, by flatplates, applied to opposing sides of the so-treated laminate results ina random arrangement of the internal cells in the partially foamedconstruction wherein voids and irregularities exist as illustratedgraphically in FIGURES 1 and 2 of the drawings. In this continuousprocess, the partially foamed continuous sheet or strip of the sandwichstructure is subjected to recompression which distorts the cells. Thisis followed by an expansion zone, shown at the right of the drawing,wherein the cells become oriented essentially into parallel elongatedrelationship normal to the opposite faces of the sandwich.

FIGURES 7 and 8 show the invention as applied to a single ply ofthermoplastic resin which may be formed, for example, as described inthe Hardy and Russell application, Ser. No. 272,540, now Patent No.3,262,625, aforementioned as shown in FIGURE 7. In the latter case, theinitial partial foaming may be effected by a partial steeping of thethermoplastic resinous article following heat treatment then followed bya period of expansion and further followed by compression by applicationof pressure as previously described in connection with a twoplypartially foamed sandwich construction. The arrangement that will beobserved from FIGURE 8 is essentially similar to that illustrated byStage 4 of FIGURE 5 and by the last stage of expansion in the continuousprocess illustrated in FIGURE 6 of the drawings, except that therecompression is applied to a structure where one side only has beenpartially foamed. FIGURE 9 shows a membrane for recompressing anirregular shaped object, while FIGURE 10 is the recompressed irregularshaped object itself.

As described in the Hardy and Russell applications, Ser. Nos. 272,540,now Patent No. 3,262,625 and 548,905, the invention is applicablevirtually to any thermoplastic material amenable to the partial steepingprocedure including but not limited to styrene homopolymers, rubbermodified styrene polymer blends, PVC homopolymers,acrylonitrile-butadiene-styrene terpolymers, mechanical polyblends andother modified graft and mechanically blended styrene polymers, vinylhalide, vinylidene halide, vinyl acetate, cellulose ester, ethylcellulose, acrylic acid esters, methacrylic acid esters, acrylonitrile,ethylene, propylene and higher olefins, isobutylene, halogenatedolefins, as well as copolymers, interpolymers, graft polymers, andchlorinated and chlorosulfonated polymers of the monomers correspondingto the above-mentioned polymeric products and mixtures of the same. Aparticularly useful material for forming articles such as containers isa rubber-modified polystyrene or polystyrene which preferably hasincorporated therein a rubber compound grafted onto the molecule.Another particularly useful material is a terpolymer polyblend system ofacrylonitrile, butadiene and styrene. It is not necessary that both ofthe opposing sheets be of the same material. In most instances,structures having unusual properties can be obtained by contacting sheetmaterials of difierent compositions to obtain the needed propertiesresulting from the combination of the two compositions. Carried further,it is also possible to join more than two members, each of which can beof different composition if desired to produce structures having unusualproperty combinations. Generally, when more than two sheet members areinvolved in the structure, the center member has both sides or surfacesin the foamed state. In fact, the internal sheet member may be eitherpartially or entirely foamed.

The medium which is absorbable by the plastic is considered to be anymaterial which is (l) absorbable by the plastic being processed and (2)convertible to a gaseous state at a temperature below 150 F. above thesoftening temperature of the plastic. Preferably, the

medium will contain or be a solvent for the plastic and may possiblyinclude a non-solvent to regulate or slow the rate of absorption and/orpartial solution of the plastic. For the sake of clarification, the termsolvent also includes those materials in which the plastic is consideredslightly soluble. The intent in the choice of the medium is to utilizematerials which after exposure to the plastic will cause the plastic tofoam on the subsequent application of heat.

The choice of the medium which may be employed will depend primarily onthe type of plastic material which is to be steeped. For example,ethylene oxide or dichlorofiuoromethane is considered quite suitable forthe acrylonitrile-butadiene-styrene terpolymers whiletrichlorofluoromethane is less desirable. On the other hand,trichlorofluoromethane is considered quite suitable for polystyrenehomopolymers and rubber modified polystyrene interpolymers. Among thesolvents which find application in the present process, there may belisted trichlorofiuoromethane, sulphur dioxide, dichlorofiuoromethane,methylene chloride, ethylene oxide, methyl formate, butadiene, acetone,dichloroethylene, carbon tetrachloride, dichlorofiuoromethane, methylsulfide, methyl ethyl ketone, benzol, chloroform and the like. Among thematerials which sometimes find utility as non-solvents or solventsdepending on the type of plastic, there may be included methanol,ethanol, n-pentane, isopentane, hexane and the like.

The exposure of the plastic to the medium generally requires only a fewseconds to a few minutes for optimum absorption depending for the mostpart on the depth of foam desired, the type of medium used, and therespective temperatures of the medium and plastic article. In general,the time of exposure to the medium is directly proportional to thesquare root of the depth of absorption into the plastic. The time ofexposure then will always be less than that which would result incomplete absorption throughout the plastic layer. As previouslyindicated, this operation may be carried out by any suitable method suchas immersion of the plastic article in a liquid medium or even bysubjecting the plastic article to solvent vapors in a vessel which mayor may not be pressurized.

In general, the plastic structure is aged a short interval of timebetween contact and heating to allow deeper diffusion of the mediumwithin the depth of the plastic material. If desired, the temperature ofthe material may be elevated somewhat to increase the rate of diffusion.On the other hand, if the temperature is increased significantly, it maybe desirable to increase the pressure on the opposing sheets to preventpremature expansion.

After this aging interval, the plastic assembly is heated to foam thesteeped portions of the member(s). The heating means utilized may varyand the final results being affected by the uniformity of heating, rateof heating and temperature level at which it is conducted.

The products of this invention are formed from substantial thermoplasticmaterials. The structures which may be fabricated from the practice ofthis invention are especially useful for panel applications requiringhigh insulating characteristics and good structural properties. Inaddition, the panel members are light-weight and have a variety of usesin the build-ing trade. Furthermore, the panel members themselves can beheated and molded to produce a variety of shapes such as cups,containers, boat hulls, enclosures, structural members, etc.

EXAMPLE I A cellular core structure is prepared by exposing one side ofeach of two 48" wide x 96" long x 0.072" thick sheets of acrylonitrile;butadiene; styrene (35:15:30) material, molecular weight (Staudinger)equals 80,000 to dichloromonofluoromethane for 15 seconds after whichthe exposed surfaces of both sheets are placed in contact in a pressunder a force applied equal to 350 p.s.i. This pressure is maintained ata temperature of 240 F. for

9 minutes and 30 seconds after which the press is opened gradually to atotal distance of 0.30" measured between press plate surfaces. When thefoam has expanded fully, the full pressure is again applied for 15seconds. The press is allowed to open to the set .30 and the panel iscooled and removed. Inspection of the finished part discloses unfoamedtop and bottom layers having a total thickness of approximately 0.060"and a cellular core approximately 0.l80" thick and 9 lbs/cu. ft. indensity. Examination of the cells indicates that for the most part theyare columnar with the long axis extended generally perpendicular to theplate of the composite.

A second structure is prepared similarly to the first structure with theexception of the original pressure cycle is maintained for 10 minutes,the press is allowed gradually to expand to 0.30" measured between pressplatens. Inspection discloses a structure similar to the above but ofmore irregular and coarser cell size.

Both the recompressed cellular core structure and the normal laminateare subjected to a series of compression tests producing an average testresult of 400 p.s.i. for the recompressed cellular cored structure and250 for the normal laminate. In other words, a significant improvementin compression strength is attained. Furthermore, shear and bendingtests indicate that improvements in these respective properties are alsoobtained on the recompressed cellular core structure when compared inrelation to the normal laminate.

EXAMPLE II The procedure of Example I is repeated except that styrenehomopolymer molecular weight (Staudinger) equals 90,000, is used inplace of the acrylonitrile; butadiene; styrene material of Example I andthe material is exposed to trichlorofluoromethane for a period of 60seconds as opposed to exposing the material to dichloromonofluoromethanefor seconds and subsequent multiple expansion employed. Substantialimprovements similar to those of Example I are obtained.

As indicated above, the multiple compressed cellular core structures ofthe present invention are superior in terms of compression bending andsheer strength properties over the normal structure which are preparedaccording to the previous methods described. Furthermore, nodiscontinuity is noted in the transition area between the unfoamedportions of the cellular cored structure as would be evident in thenormal sandwich structures.

Although the above examples, for illustrative purposes, describe thepreparation of sandwich structures, the invention is likewise applicableto reorientation of cell structure in partially foamed thermoplasticmaterials wherein the pneumatogenic material is applied to a singlesurface of the thermoplastic substrate as described in the Russell andHardly application, Ser. No. 272,540. Also, the invention contemplatesthe initial preparation of a partially foamed panel structure either insandwich form or with a single foam surface and the panel structure thenis subsequently subjected to recompressive treatment to produce avariety of shapes and wherein during the establishment of such shapesthe foaming is completed as the said panels conform to the contours ofthe mold.

Regarding the equipment used to carry out the present process, all thatis essential is that a volatile medium which is absorbed by the plasticbe allowed to contact the surface of the plastic for predeterminedlength of time and a source of heat to expand the member and a source ofpressure to allow the multiple expansion procedure. FIGURE 2 illustratesa bottomless chamber containing a liquid volatile substance which isheld in the chamber by the sheet passing in contact with the chamberbottom. It is, of course, obvious that other means may be used such asvapor, coating methods and even folding the sheet to retain a pool ofliquid during this step.

What is claimed is:

1. A method of manufacturing structures having cellular cores whichcomprise exposing at least one surface of a thermoplastic surface to amaterial which is (l) absorbable by the plastic being processed and (2)convertible to a gaseous state at a temperature below 150 F. above thesoftening temperature of the plastic, for an interval of time less thanthat which would result in complete absorption throughout the plasticlayer, intimately contacting at least a portion of said surface with asecond plastic surface, placing the resulting assembly in confiningmeans designed to control the rate of foam expansion, subjecting saidresulting assembly to a temperature between the gas transitiontemperature of the medium which is absorbable by the plastic and atemperature which is 150 F. above the softening temperature of theplastic, allowing the plastic exposed to the absorbable material toexpand, thereby forming a plastic structure having an internal cellstructure, reapplying said confining means to the assembly whilesubjecting said assembly to a temperature between the gas transitiontemperature of the medium and a temperature which is 150 F. above thesoftening temperature of the plastic, then releasing the plasticstructure from the confining means, thereby yielding a plastic structurehaving a subdivided and reoriented internal cell structure.

2. A method of manufacturing structures having cellular cores and atleast one solid surface which comprises exposing one solid surface of athermoplastic article to a material which is (l) absorbable by theplastic being processed and (2) convertible to the gaseous state at atemperature below 150 F. above the softening temperature of the plasticfor an interval of time less than that which would result in completeabsorption throughout the plastic layer to partially foam said surface,allowing expansion of said partially foamed surface to a thickness offrom about 10 to percent of the final thickness, applying pressure tosaid article to compress said partially foamed surface to an extent ofabout 10 to 100 percent of the initial thickness of said partiallyfoamed surface, releasing said pressure and allowing expansion of saidpartially foamed surface to the extent desired in the finished article.

3. A method as set forth in claim 2, wherein the medium absorbable bythe plastic is trichlorofiuoromethane.

4. A method as set forth in claim 2 wherein the medium absorbable by theplastic is dichloromonofluoromethane.

5. A method as set forth in claim 2 wherein the medium absorbable by theplastic is a mixture of a solvent and a non-solvent for the plastic.

6. The method according to claim 2 wherein the solid plastic form is aninsulating and structural panel.

7. The method according to claim 2 wherein the solid plastic form is aninsulating panel.

8. The method according to claim 2 wherein the solid plastic form is astructural panel.

9. A method for producing a foamed surface on styrene-based polymerarticles which comprises the steps of (l) shaping a solid plastic form,

(2) steeping the plastic form in a medium absorbable by the plastic andconvertible to the gaseous state at a temperature below F. above thesoftening temperature of the plastic until said medium is absorbedpartly through the plastic,

(3) removing the plastic form from said medium,

(4) heating the plastic form to produce a foamed covering having aninternal cell structure at the area heated which had been in contactwith said medium,

(5) applying pressure to the foam covering,

(6) releasing said pressure, and

(7) allowing expansion of foamed covering to the extent desired in thefinished article thereby reorient- 8 ing the internal cell structure ata direction normal FOREIGN PATENTS to said surface.

References Cited 591,621 1/ 1960 Canada.

UNITED STATES PATENTS PHILIP E. ANDERSON, Primary Examiner 2,769,20511/1956 Pflcumcr 26455 5 3,082,483 3/1963 Bickford 264-321 3,238,5653/1966 Jacobs 264-321 XR 114-.5; 161-16(), 161; 220-83; 229--3.5;264-47, 53,

3,262,625 7/1966 Russell et a1. 26453 XR 55, 321

